/outsystems-custom-code

name: outsystems-custom-code
description: '[Beta] Reference + workflow for building, testing, and deploying C# class-library "External Logic" libraries for OutSystems Developer Cloud (ODC). Covers SDK attributes ([OSInterface] / [OSAction] / [OSParameter] / [OSStructure] / [OSIgnore]), supported data types, optional parameters, logging via ILogger, project structure, runtime constraints (95s execution / 1 GB memory / 5.5 MB payload), unit + container integration tests, and deployment via the OutSystems MCP. Use when the user asks to "build a C# library for OutSystems", "create ODC external logic", "write custom code for ODC", "deploy an external library", "publish C# code to OutSystems", or names ODC External Logic / external library / OSInterface / OSAction directly. Not for OutSystems 11 (O11) or Service Studio.'
license: MIT
compatibility: Ships for Claude Code, Cursor and Kiro. Needs a shell with the .NET 10 SDK, and Docker for the integration tests, so Claude Desktop's Chat tab, which has no shell, can't run it. Deployment requires the `outsystems` MCP server connected and authenticated, and builds on the main `outsystems` skill for the upload, polling and publish rules. Python is NOT required by this skill.
metadata:
  version: "1.3.0"
  author: outsystems-r-and-d
  maturity: beta

**Beta Feature.** This skill is a Beta Feature: a non-final OutSystems capability provided to collect customer feedback. It can change significantly, including through breaking changes, or be discontinued. The first time you use this skill in a conversation, tell the user it's a Beta Feature and share <https://www.outsystems.com/legal/beta-features-agreement>.

OutSystems ODC External Library Skill

This file describes how to create, build, test, and deploy C# libraries for the OutSystems Developer Cloud (ODC) External Logic feature.

**Load the main `outsystems` skill before uploading: upload, polling and publish rules come from it** (read the live tool catalog, poll long-running operations to terminal, confirm before every tenant write). The main skill's upload, polling and publish rules apply, and this skill's two confirmations (see [Upload and publish](#upload-and-publish)) always apply on top of them. For building and packaging the library, follow this skill.

---

Project Overview

OutSystems ODC External Logic allows extending ODC apps with custom C# code. The C# class library project is built locally, published as a ZIP file to the ODC platform. OutSystems generates a wrapper around the code that exposes the decorated interfaces as server actions consumable by ODC applications.

---

Runtime Constraints

These are hard limits enforced by the ODC platform. Never design functions that approach or exceed them:

| Constraint | Limit | |---|---| | Max execution duration | **95 seconds** per function call | | Max memory | **1,024 MB** | | Max ephemeral storage | **512 MB** | | Max payload (inputs + outputs combined) | **5.5 MB** |

For large data: stream in chunks, use references/URLs instead of raw bytes.

---

Project Setup

Target Framework

• Use **.NET 10**. Do **not** use .NET 6 or .NET 8: .NET 8 support is ending, and after that ODC

refuses to upload, update, or install a library that targets it.

• Project type: **Class Library** (`<OutputType>Library</OutputType>`).
• If the project targets an older .NET, update it to .NET 10.

Before you build

If `dotnet --list-sdks` shows no 10.x SDK, or `docker info` fails, tell the user and stop at the step that needs it: the build and unit tests need the SDK, the container tests need Docker.

Required NuGet Package

<PackageReference Include="OutSystems.ExternalLibraries.SDK" Version="1.7.0" />

Always use the latest stable version. Check: https://www.nuget.org/packages/OutSystems.ExternalLibraries.SDK

Minimal `.csproj`

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>net10.0</TargetFramework>
    <Nullable>enable</Nullable>
    <ImplicitUsings>enable</ImplicitUsings>
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="OutSystems.ExternalLibraries.SDK" Version="1.7.0" />
  </ItemGroup>
</Project>

Note: the SDK package itself may only ship `net8.0`/`netstandard2.0` assets (no `net10.0`-specific build) — that's fine, NuGet resolves the closest compatible asset and it runs correctly under a `net10.0`-targeted project.

---

SDK Attributes – Core Concepts

`[OSInterface]` – Exposes the interface to ODC

Applied to a **public interface**. Every method on this interface becomes a server action in ODC.

using OutSystems.ExternalLibraries.SDK;

namespace MyCompany.MyLibrary
{
    [OSInterface(
        Name = "MyLibrary",
        Description = "Short description of what this library does."
    )]
    public interface IMyLibrary
    {
        [OSAction(Description = "Greets a person by name")]
        string Greet(
            [OSParameter(Description = "Name to greet")] string name
        );
    }
}

• `Name`: The library name in ODC. It also decides whether an upload creates a new library or a new revision, so always set it. Required for ODC Forge submission.
• `Description`: Shown in ODC Portal. Required for Forge submission.
• `IconResourceName`: the embedded resource name of the library's PNG icon. Required for Forge submission. Add the icon to the project as an `EmbeddedResource` item and set `<RootNamespace>MyCompany.MyLibrary</RootNamespace>` in the `.csproj`; the resource name is then the root namespace, the folder and the file name, joined with dots (for an icon in `Resources`, `MyCompany.MyLibrary.Resources.` followed by the file name). Keep these out of the minimal `.csproj`: the build fails when the icon file doesn't exist.

`[OSAction]` – Customise action metadata (optional)

Applied to individual interface methods to override defaults.

[OSAction(
    Description = "Validates an IBAN and returns structured result.",
    Retur